THE
CELL AND ITS FUNCTION
Each one of the 50 to 100 trillion cells in the
human body is a living is a living structure that can survive indefinitely
and,in most instances, can even reproduce itself, provided its surrounding
fluids contain
appropriate nutrient. To be able to understand
the function of organs and the other structures of the body, it is essential
that we first understand the basic organisation of the cell and the function of
its
various parts.
1.0 ORGANISATION OF THE CELL
A typical cell, as seen by the light microscope
, has two major parts; the nucleus and the cytoplasm. The nucleus is separated from the
cytoplasm
by a nuclear membrane, and the cytoplasm is separated from the surrounding
fluids by a cell membrane. The different substances that make up the cell are
called protoplasm. Protoplasm is
composed mainly of five basic substances: water,
electrolytes, protein, lipids,and carbohydrates.
1.1 THE
PHYSICAL STRUCTURE OF THE CELL
The cell
is not merely a bag of fluid, enzymes, and chemicals; it also contains highly
organized physical structures most of it
called organelles, and the physical nature of these is
equally as important to the function
of the
cell as the cell's chemical constituents. An example would be the mitochondria
which the cell gets 95% of its energy from. Some principal organelles or
structure of the cell includes the cell
membrane, nuclear membrane, endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, and centrioles.
1.2 THE
MEMBRANOUS STRUCTURES OF THE CELL
Essentially all organelles of the cell are lined by membranes
composed primarily of lipids and proteins. These
membranes include the cell membrane, the nuclear membrane,
the
membrane of the endoplasmic reticulum, and the membranes of the
mitochondria, lysosomes, Golgi apparatus,e.t.c. The cell
membrane,which completely envelops the cell, is a very thin, elastic
structure only 7.5-10 nanometers thick. It is
composed entirely out of proteins and lipids.
1.3
FUNCTIONAL SYSTEMS OF THE CELL
There
are several representative functional system of the cell that make it a living
organism and we are going to discuss some of them.
1.3.0
INGESTION BY THE CELL ENDOCYTOSIS
If a cell is to live and grow, it must obtain nutrient and other substances from the sorrrounding fluids.Most substances passes through the cell membrane by diffusion and active transport.
However large particles enter the cell by a specialized function of the cell membrane called Endocytosis. The two principal forms of Endocytosis are Pinocytosis and Phagocytosis. The Pinocytosis means the ingestion of extremely small vesicles containg extra cellular fluid. Phagocytosis means ingestion of large particles, such as bacteria, cells or portions of degenerating tissues.
Pinocytosis: Pinocytosis occurs continually at the cell membranes of most cells but especially rapidly in some cells. As an instance, it occurs so rapidly in macrophages that about 3% of the total macrophage membrane is engulfed in the form of vesicles each minute. The pinocytic vesicles are so small, usually only 100-200 nanometers in diameter, that most of them can be seen only with the electron microscope. Pinocytosis is the only means by which some very large macromolecules such as most protein molecules, can enter cells. As a fact , the rate at which pinocytic vesticles form is usually enhanced when such macromolecules attach to the cell membrane.
Phagocytosis: Phagocytosis occurs in much the same way as pinocytosis except that it involves large particles rather than molecules. Only certain cells have have the capability of phagocytosi most notably the tissue macrophages and some of the white blood cells. Phagocytosis is initiated when proteins or large Polysaccharides on the surface of the particle that is to be phagocytized-such as a bacterium, a dead cell, or other tissue debris-bind with the receptors on the surface of the phagocyte.
1.3.1 DIGESTION OF FOREIGN SUBSTANCES IN THE
CELL - FUNCTION OF THE LYSOSOMES
The lysosomes can the called the digestive organ of the cell. Almost immediately after a pinocytic or phagocytic vesicle appears inside a cell, one or more lysosomes become attached to the vesicle and empty their hydrolases into the vesicle, thus a digestive vesicle is formed in which the hydrolases begin hydrolyzing the protein, glycogen, nucleic acids, mucopolysaccharides, and other substances in the vesicles. The product of the digestion which are amino acids, glucose,...e.t.c, can be then diffused through the membrane of the vesicle into the cytoplasm. What is left of the digestive vesicle, called the residual body represents the undigestible suubstances, which in most instances can be excreted through the cell membrane by a process called exocytosis.
The lysosomes
are also responsible for the REGRESSION of TISSUES and AUTOLYSIS of
CELLS.
1.3.2 EXTRACTION OF ENERGY FROM
NUTRIENTS - FUNCTION OF THE MITOCHONDRIA
The principal substances from which cells extract energy are oxygen and one or more of the foodstuffs - carbohydrates, fats, and proteins. All cabohydrates and proteins and fats are converted into glucose and amino acid and fatty acids respectively before the reach the cell - (in the human body). The glucose, amino acids and fatty acids, then enter the cell. Inside the cell the foodstuff react chemically with oxygen under the influence of various enzymes that control their rates of reactions and channel the energy that is released in the proper direction.
Another
function of the cell is the SYNTHESIS and FORMATION of CELLULAR
STRUCTURES by the ENDOPLASMIC RETICULUM and GOLGI APPARATUS.
References: GUYTON Textbook of Medical Physiology eighth edition.